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IS MISTLETOE THE KISS OF DEATH?
by
H. A. (Joe) Pase III
Texas Forest Service
It's a Christmas tradition. A man has the privilege of kissing a lady who happens to
be standing under a sprig of mistletoe, a plant which is said to symbolize peace and love.
The danger associated with this tradition is that the lady may take offense at the man's
affection! Have you ever given or received a kiss under the mistletoe? Even though
mistletoe is a popular Yuletide decoration, all parts of the plant have poisonous properties
and should never be eaten. Except for Christmas holidays, mistletoe is probably not a
particularly popular plant, especially if it is growing in a valuable shade or ornamental tree
in your yard.
Is mistletoe becoming a serious problem in East Texas? A definitive answer to that
question is not possible because no surveys have been conducted to determine whether
mistletoe incidence is increasing or decreasing. Generally speaking, mistletoe is not
considered to be a serious pest of trees, so don't kiss your tree good-by if it is infected with
mistletoe. When heavy infection occurs, the mistletoe becomes an additional stress factor to
the tree and may contribute to poor tree health. It is unlikely that mistletoe kills trees
directly, but tree branches may die as a result of mistletoe infection. Effective control of
mistletoe is difficult to achieve.
American mistletoe (genus Phoradendron) is a persistent, evergreen, photosynthetic,
seed-producing plant that is parasitic on certain woody plants, primarily hardwood or
broadleaf trees. It is particularly conspicuous on hardwoods after leaf fall. The mistletoe
derives water and mineral nutrients from the sap of its host plant. Because the leaves of the
mistletoe plant contain chlorophyll (green color), the plant can produce its own food through
photosynthesis using water and minerals derived from the tree that supports it. The leaves
are leathery and occur opposite each other on the mistletoe stem. The plant requires direct
sunlight for best development which explains why it is usually found high in the crown of
tall trees. Mistletoe may change from green to a greenish-yellow color during the winter
months, but this is not an indication that the plant is unhealthy.
The best indication of mistletoe infection is the presence of dense clusters of
vegetation in the crown of host trees. The evergreen mistletoe plant is most easily
recognized in the winter months when deciduous trees have dropped their leaves. Trees
vary in their susceptibility to mistletoe with water oak, hackberry, and elm being the most
commonly infected. Ash, beech, cherry, dogwood, sweet gum, hickory, maple, Osage
orange, persimmon, sassafras, walnut, sycamore, and willow also may be infected. It has
been reported on 110 different species of trees in the eastern United States.
Mistletoe in East Texas is a dioecious plant -- that is, male and female flowers are
borne on separate plants. The flowers are small and creamy-white in color.
Only the female flowers produce seeds which are white and embedded in a sticky,
gelatinous pulp enabling the seeds to adhere to the bark of trees. The seeds are commonly
distributed by sticking to the beaks and feet of birds or by bird droppings after passing
through the bird's digestive system. Under favorable temperature and moisture conditions,
the seeds germinate almost anywhere, but trees will only become infected when seeds
germinate on the thin bark of small branches. Successful infection of a host tree occurs
when the mistletoe seed germinates and a root-like structure (called a haustoria) penetrates
the bark through a lenticel or bud.
The aerial portion of the plant develops very slowly the first year, growing less than
one-half inch. Under ideal conditions, mistletoe may develop an aerial spread of three feet
in six to eight years. The longevity of mistletoe plants seems to be limited only to the life of
the host tree with some plants living for more than 100 years. On the average, the aerial
portion of a single mistletoe plant survives less than eight years, being easily broken off by
storms. Destruction of the aerial portion of the plant usually stimulates the development of
dormant buds and multiplies the presence of the plant on its host.
Mistletoe has few natural enemies, and effective control is very difficult to achieve.
Where feasible, the homeowner can take steps to minimize mistletoe problems in trees.
Mistletoe plants mature in two to three years, so mechanically removing the aerial portion of
the plant before it matures and produces seeds can be of some benefit. Small, infested limbs
can be removed by pruning, if they can be reached. Mechanical removal of the aerial
portion of the plant on large limbs may also be helpful. Removing part of the wood where
the mistletoe attaches to its host should be avoided as this usually causes more damage than
the mistletoe itself. Keep in mind that mechanical removal must be done repeatedly because
new sprouts will grow from the mistletoe imbedded in the wood of the host tree.
Mechanical removal is suggested only if it can be done safely and economically.
In theory, covering the aerial portion of the plant with a black plastic bag would
block sunlight and the plant would eventually die. However, in practice, covering the plant
high in the top of a tree can be difficult and dangerous. In addition, plastic bags tend to
deteriorate over time; and the bags would be unsightly, especially in the winter months
when trees are bare. Also, applying a herbicide to the mistletoe during the winter when the
host tree has no leaves has met with limited success. However, because it is so easy to
damage the host tree and other nearby plants, using a herbicide is not recommended. A
plant growth regulator called ethephon (Florel7 Fruit Eliminator) is the only product
registered in the USA for the control of mistletoe on deciduous trees. Local nurseries or
“feed and seed” stores may handle Florel7.
So, what do you do with mistletoe? Enjoy it, because you probably won't be able to
successfully eliminate it from your tree. Try to get your sweetheart to stand with you under
a mistletoe-infected tree, and steal a kiss!